The default raid frames can be sorted in several different ways, depending on the user's game settings. Fortunately, you don't actually need to worry about how they are sorted. The simplest solution would be to simply hook the function that each frame calls when it updates its own absorb display, and update your attached display frame at the same time:
Code:
hooksecurefunc("CompactUnitFrame_UpdateHealPrediction", function(frame)
-- Find out which unit the frame is displaying:
local unit = frame.displayedUnit
-- Do stuff here.
end)
Without seeing your code, or even knowing what you're adding to the frames -- is it a texture? a font string? a status bar? something else? -- it's hard to give any more relevant suggestions, though.